Bandgap tuning is at the core of current materials research and optoelectronic device applications. Through successful tuning of the bandgap in semiconductors, bandgap—tailored heterostructures including two-dimensional electron gas and tunneling structures were realized. Such advancements offered a greater understanding of physics regarding quantum electrodynamics and stimulated emergence of many related devices. Moreover, the ability to tune the bandgap is becoming increasingly important for developing highly efficient solar cells and transparent conducting oxides.
For conventional III-V and II-VI semiconductors, simple bandgap tuning has been extremely successful, leading to realization of the structures mentioned above. For example, GaAs has a bandgap of 1.42 eV, which can be continuously tuned down to 0.35 eV or up to 2.12 eV by substituting In or Al for Ga, respectively. Such simple alloying results in a bandgap spectrum of larger than 1 eV.
On the other hand, recent breakthroughs in complex oxides have provided an opportunity to incorporate our understanding of semiconductors into the exotic physics of transition metal oxides (TMOs). For example, the observation of quantum transport behaviors in several complex oxides manifest a substantial improvement of oxides in terms of their quality and leading to properties that were thought to be unique to semiconductors. However, substantial and controllable bandgap tuning has yet to be achieved in TMOs, despite intense effort. To tune the bandgap of TMOs, one might consider modifying or substituting the transition metal with another element, because the rigid nature of the bandgap originates mostly from the strongly localized character of the d -electrons. However, what has been observed is that the fascinating physical properties of TMOs arising from the d -electrons disappear with the modified bandgap.
Such difficulties have hampered the recent searches for more efficient transparent conducting oxides and low-bandgap photovoltaic oxides. Ferroelectric oxides, in particular, are attracting renewed attention owing to their inherent built-in potential arising from the spontaneous polarization, which could be used to efficiently separate photon-induced electron-hole pairs.
Possibilities of oxide optoelectronics are being ceaselessly investigated as a way to overcome the eminent issues of Si-based semiconductor electronics. The strong correlation of d-electrons and oxygen in transition metal oxides results in exotic physical properties and, therefore, emergent phenomena such as superconductivity, colossal magnetoresistance, and ferroelectrics are observed. However, current oxide materials also have their own drawbacks to catch up the semiconductors. One of the most important characteristics required for the oxides is the ability to systematically tune the band gap. The band gap in semiconductors can be continuously controlled over an electron volt (eV) by simple doping, enabling band gap engineering relevant for numerous devices including high electron-mobility transistor and tunneling devices. However, the band gap in transition metal oxides cannot be tuned easily, due to the rigid nature of d-electron bands. Up to this point, less than 0.2 eV band gap spectrum has been achieved for transition metal oxides. The present invention is directed to the need for methods and materials to easily and systematically control the band gap of complex transition metal oxides.